1. Field of the Invention
The present invention relates to a power transducer.
2. Description of the Related Art
Inverters, a form of electrical power transducer, have been widely adopted in industries and also used in household electric appliances as rotational velocity control devices for AC motors. When the rotor of an AC motor is decelerated, the rotational energy of the AC motor during the deceleration is accumulated as electrostatic energy in a smoothing capacitor located in the DC intermediate circuit of its transducer. However, voltages may increase at both ends of the smoothing capacitor due to its small capacitance. This puts into operation the overvoltage protection circuit provided in the DC intermediate circuit of the transducer, bringing the transducer to a halt.
For this reason, a power transducer is provided with a regenerative braking resistor in its DC intermediate circuit so that the regenerative braking resistor consumes the deceleration rotational energy of the AC motor as thermal energy.
The rated currents of the regenerative braking resistor and of power semiconductors in the regenerative braking circuit have upper limits. If used beyond the limits, the regenerative braking resistor and the power semiconductors receive extremely high voltages: they may be thermally damaged due to electrical heat generation. Thus, a maximum duty factor, a quantitative attribute, is predefined for the regenerative braking resistor (the maximum duty factor is hereinafter referred to as % ED).
Therefore, the power transducer manages the cumulative operating time of the regenerative braking circuit provided in its DC intermediate circuit. When the cumulative operating time percentage exceeds the % ED of the regenerative braking resistor, the power transducer disconnects a switching element in the regenerative braking circuit provided in the DC intermediate circuit to cut off the current flowing in the regenerative braking resistor, thus protecting the regenerative braking resistor from thermal destruction due to electrical heat generation.
However, no strict definition has been given to the permissible duty factor % EDpm of the power semiconductor (as a switching element) of the regenerative braking circuit provided in the DC intermediate circuit in the power transducer. Only the % EDR is determined based mainly on the correlation with the regenerative braking resistor.
A power semiconductor used for the power transducer, such as IGBT, generates heat because of electrical loss generated during electric power conversion. Therefore, operating the power transducer beyond the semiconductor's operating limit temperature may result in thermal destruction and deactivation. Therefore, the power transducer includes a cooling fin and a cooling fan for cooling the power semiconductor. Heat is conducted from the power semiconductor to the cooling fin, and the cooling fan sends air to the cooling fin for thermal exchange to allow heat radiation.
Paragraph 0007 of Japanese Patent No. 3648932 describes that the voltage-type power transducer comprises: a series circuit connected at both ends of the smoothing capacitor, the series circuit consisting of a braking resistor and a transistor switch; a voltage detecting circuit which closes the switch when the voltage at both ends of the smoothing capacitor exceeds a predetermined value; a totalizing circuit which totalizes time intervals during which the switch is closed within a predetermined time period (T) based on a start command from outside, and outputs the total time (Σt); a usage rate calculation circuit which calculates the usage rate (X %: X=(Σt/T)*100) of the braking resistor from the predetermined time period (T) and total time (Σt); and a display circuit which displays the usage rate (X %) when the predetermined time period (T) has elapsed.
Further, JP-A-5-168287 and JP-A-10-229607 describe that overload protection and overheat protection are performed for the regenerative braking resistor.